1. Field of the Invention
The present invention relates to an active matrix type of liquid crystal display apparatus and a driving method for the same. In particular, the present invention relates to a driving technique to for improving the quality of a moving picture image.
2. Description of the Related Art
FIG. 8 is a perspective figure showing a configuration of the active matrix type liquid crystal display apparatus of the related art. As shown in FIG. 8, the display apparatus of the related art has a panel structure comprising a pair of insulator substrates 101, 102 and a liquid crystal 103 held in between those two substrates. A pixel array unit 104 and a drive circuit unit are fabricated and integrated on the insulator substrate 101 disposed at the lower side. The drive circuit unit consists of a line drive circuit 105 and a column drive circuit 106. A terminal unit 107 for an external connection is fabricated on an upper part of a peripheral area of the insulator substrate 101. The terminal unit 107 is connected to the line drive circuit 105 and the column drive circuit 106 via wiring 108. Gate wiring 109 in a line form and signal wiring 110 in a column form are fabricated in the pixel array unit 104. A pixel electrode 111 and a thin film transistor (TFT) 112 for driving the pixel electrode 111 are fabricated at an intersection of the gate wiring 109 and the signal wiring 110. A gate electrode of the thin film transistor 112 is connected to a corresponding gate wiring 109, a drain region to a corresponding pixel electrode 111, and a source region to a corresponding signal wiring 110. The gate wiring 109 is connected to the line drive circuit 105, and the signal wiring 110 is connected to the column drive circuit 106.
Due to technical advancements in devices, process and fabrication, the active matrix type liquid crystal display (LCD) apparatus with a size up to a twenty inch class may now be realized now. In addition, displays having brighter and fine picture quality are being developed. Furthermore, improvements are also being made in order to solve problems relating to the narrow viewing angle of the liquid crystal display (LCD), which is considered one of the drawbacks in the LCD, by implementing technologies such as switching of liquid crystal molecules with an electric field along a substrate plane direction (so called in-plane switching), by combining of a liquid crystal alignment direction division and a vertical alignment (so called multiple vertical alignment), or by using a phase shift correction film. The problems related to the viewing angle are such that the viewing angle of the LCD in which more than a reasonable contrast can be obtained is narrower than that of a CRT, and a negative-positive inversion may occur locally for a gray scale image display. Furthermore, according to advancements in production technologies, the cost of the LCD has been cut considerably such that even a twenty inch class LCD television is now coming into practical use. With the use of these technologies mentioned above, a picture quality of the LCD has become comparable and even superior to that of the CRT as far as a still picture image is concerned.
However, various drawbacks of the LCD remain to be solved. One is the image quality of a moving picture. That is, the LCD may not be able to generate clear outlines of moving pictures and the moving pictures displayed on the LCD screen may smear. For example, in an extreme case, a trailing tail image of a pitched ball may be appear on the LCD screen during a baseball game broadcasting. Such an extreme case is now being resolved due to technical advancements in liquid crystal materials.
Quantitatively, a total period (i.e. response time) of a rise time for horizontally oriented liquid crystal molecules to be risen by a certain electric field, and a fall time for the risen liquid crystal molecules to go back to their original orientation with null electric field is reduced to as short as about 30 msec due to technical improvements. Presently, liquid crystal molecules are driven to rise or fall at the beginning of every 33.3 msec frame period for the LCD with a 30 Hz frame frequency. In other words, the response characteristic of the LCD has been improved such that the liquid crystal molecules can be driven to follow the frame frequency without any difficulties.
However, the problem of clarity of the moving picture outlines remains unsolved. This problem may not be improved even by further development of liquid crystal materials with shorter response times, nor by further improvements in orientation technology. An underlying cause of the problem is based on a fundamental principle of the active matrix type LCD, and reported in “Improving the Moving-Image Quality of TFT-LCDs” at the International Display Research Conference (IDRC), 1997.
FIG. 9 is a schematic view illustrating the problem of moving image quality of an active matrix type LCD of the related art. Image data for each frame is shown on the left hand side of FIG. 9, and the visual picture appears on a display screen (hereafter, called visual screen image), as shown on the right hand side of FIG. 9. An image data SIG1 at a frame 1 shows, for example, an alphabetical character of X. The next frame (frame 2) also shows the same character X except with a slight shift toward the right hand side. The bottom frame (frame 3) also shows the character X shifting toward a bottom-left direction. On the other hand, residual images (shadows) may appear in the visual screen image, which are actually recognized by human eyes, when the frame changes from frame 1 to frame 2, and when frame 2 changes to frame 3. Because of these shadows, the problem surrounding the capability of displaying moving images on active matrix type LCDs of the related art remains.
FIG. 10 is a waveform diagram schematically showing a driving method of the active matrix type LCD of the related art shown in FIG. 9. In general, the LCD is driven in an AC mode. Accordingly, each frame (for example frame 1) is divided into a field 1 and a field 2, and the LCD is interlace driven. In frame 1, image data SIG1 is written into liquid crystal pixels for a period of field 1 and field 2. In the next frame (frame 2), image data SIG2 is similarly written into the liquid crystal pixels for a period of field 1 and field 2. The image data written into each liquid crystal pixel is kept during the frame pertaining to the active matrix type driving method. When the frame is changed to the next frame, the image data is re-written instantaneously. Namely, the image data is suddenly switched between frame 1 and frame 2, whereby causing the residual image phenomenon. Human eyes recognize the residual image during switching of the frames in which, for example, the liquid crystal pixel written-in the white at frame 1 is switched to the black at frame 2.
The brightness of the image shown on the CRT screen attenuates in an order of a microsecond. In contrast, a fundamental principle of a display method for the LCD is to keep the same display image for an entire frame. The LCD displays the same image until the switching of the frames starts. This will be added to the residual image phenomenon of human eyes as described above. Accordingly, the residual image may still be recognized even after the frame has been changed, despite the ultimate advancement of the response characteristics of the liquid crystal material. This remains the fundamental problem surrounding the moving image quality of the active matrix type LCD.
To solve this problem, utilization of an “OBC mode” technique is suggested by the report mentioned above to improve the moving image quality. The OBC mode technique is a technology for cutting the residual image recognized by the human eyes and is based on the assumption that the liquid crystal response time is about 5 msec. For example, in the transmission type LCD, a back light is blinked within a single frame so as to display an image at the former part of the frame and tune the back light off at the latter part, thereby inducing a phenomenon similar to the fast attenuation of the CRT brightness. However, there are some drawbacks in this technique. For one thing, the contrast of the LCD is decreased since the blinking of the back light causes a decrease in the average luminosity and darkens the screen. Furthermore, power consumption and production costs will increase due to the intermittent drive of the back light. Furthermore, the technique can not be applied to a reflection type LCD, which is widely used in the present days. Some improvements are reported in “A Novel Wide-viewing-Angle Motion-Picture LCD”, Society of International Display, 1998 regarding problems on the back light power consumption and its application to the reflection type LCD. However, the report did not provide solutions to the problems surrounding the brightness and contrast of the LCD.